According to kellymom, it is approved by the AAP for use by nursing mothers. It's an L2.

When I checked safefetus, though, Imodium AD comes up as a C, and just Imodium comes up as a B, even though it has two ingredients while the AD has one. safefetus.com/search.php/index/search/type/1/letter/i

I would probably trust kellymom and use it, though. But that's just me.